The present invention relates to a charging device for electrophotographic image forming equipment and, more particularly, to a scorotron charging device having a main charger with discharge wires and a grid and applying voltages to the wires and the grid by respective power sources.
It is a common practice with electrophotographic image forming equipment, e.g., a copier, facsimile transceiver or laser printer to charge, before imagewise exposure, the surface or a photoconductive element or image carrier by a charging device which effects corona discharge. For the corona discharge, a voltage is applied to a discharge wire forming part of a main charger which is included in the discharging device. When the products of discharge, toner particles or similar impurities deposit on the discharge wires, the corona discharge and, therefore, the charge distribution on the surface of the photoconductive element becomes irregular. This prevents a toner from depositing in a constant amount on a latent image electrostatically formed on the photoconductive element in the event of development. A scorotron charging device is an effective implementation against such an occurrence and has a grid between the discharge wire and the photoconductive element. Specifically, a scorotron charger generally has a main charger made up of one or more discharge wires and a casing connected to ground, a grid interposed between the wires and the photoconductive element, a common power source for applying voltages to both of the wires and grid, a switch connected between the wires and grid and the common power source, a controller for controlling the switch, and a varistor intervening between the switch and the grid. When the controller closes the switch, part of the charge generated by the wires is released to ground via the casing while the other part is directed toward the grid and photoconductive element. Since a voltage controlled by the varistor is applied to the grid, the amount of charge to be deposited on the surface of the photoconductive element is also controlled. This type of scorotron charger is disclosed in, for example, Japanese Patent Laid-Open Publication No. 72177/1989.
The conventional scorotron charger in which the discharge wires and grid share a single power source has a problem that a substantial period of time is necessary for the grid voltage to rise or fall. Assuming that the timing for starting charging the photoconductive element is delayed, then a bias voltage for development will rise first and cause a toner to deposit on the surface of the element other than an image area. This not only results in the waste of toner but also increases the load on a cleaning device for cleaning the photoconductive element. On the other hand, assuming that the wire voltage rises before the grid voltage, then the photoconductive element will be charged excessively to suffer from fast fatigue or to cause a toner or a carrier to deposit needlessly on the element. Further, since the power source for applying a voltage to the grid has a high impedance, it is likely that a current fails to flow from the wires to the grid and leaks from the wires to the photoconductive element to thereby burn the element 601.